Gas chromatography techniques are used in analytic chemistry applications to separate and/or analyse components of a mixture. Gas chromatography uses a carrier gas as its mobile phase and a layer of liquid or polymer on a solid support as its stationary phase, located in a metal tube referred to as a column. Gaseous compounds in a sample being analysed interact with the stationary phase as it passes through the column with the carrier gas. Different compounds interact at different rates and elute at different times. Analysis of the response factors of the compounds allows information to be derived about the compounds.
Gas chromatography has numerous industrial applications. For example, it is used in the oil and gas industry to analyse the composition of a natural gas, which typically includes inert components and hydrocarbon components ranging from C1 to C7+: i.e. Nitrogen, CO2, methane, ethane, propane, isobutane, n-butane, isopentane, n-pentane, hexanes, heptanes and higher alkanes. In order to analyse such a sample in a practical time frame and without temperature ramping, a multi column separation technique is required. A C6+ gas chromatograph (GC) system is configured to analyse components from C1 to C5 separately, with C6 and higher compounds giving a single output. A C7+ GC system is configured to analyse components from C1 to C6 separately, with C7 and higher compounds giving a single output.
A typical three column GC design is shown in FIG. 1 generally at 100, and uses chromatograph valves 101, 102, three columns 103,104,105, a restrictor 106, a reference detector 108 and measuring detector 107 in a controlled temperature chamber 109. The detectors 107, 108 are thermistors, where resistance changes are dependent on the temperature. The reference and measuring detectors form a balanced Wheatstone bridge. Helium is the preferred carrier gas because it has high thermal conductivity, although Nitrogen, Hydrogen and Argon can also be used in special circumstances. FIG. 1 shows a flow path of the C1, C2, and C6 compounds through the third column 105.
With only carrier gas flowing across the two detectors 107, 108, the Wheatstone bridge is in balance. In the measuring detector, the sample gases passing across the thermistor cause thermal conductivity changes, which result in a change of thermistor heat exchange rate. This in turn results in a change of the temperature of the thermistor. The change of temperature results in a change of resistance in the measuring detector and unbalances the Wheatstone bridge. The magnitude of the voltage created by the unbalanced bridge and the time taken to pass through the detector then forms a response curve; the area under the curve is proportional to the amount of the component in the carrier gas stream.
Actuation of the valves controls the flow of gases in the GC. There are three important valve timings on the three-column chromatograph as follows:                1. Valve 101 is actuated to allow the heaviest component (C6+ in a C6+ GC application, or C7+ in a C7+ GC application) to be back-flushed. The back-flush is initiated after C5 and lighter components (in a C6+ GC system) or after C6 and lighter components (in a C7+ GC) are eluted from column 103 to column 104, but before the heaviest component (i.e. C6+ or C7+) leaves column 103.        2. Valve 102 is actuated to trap the light components in column 105. The valve actuation has to be after all of the C2 (ethane) is eluted into column 105 but before any C3 (propane) leaves column 104.        3. Valve 102 is actuated to allow light components to leave column 105. The valve actuation has to be after all of the middle components (C3 to C5 in a C6+ application; C3 to C6 in C7+ application) clear the measurement detector.        
During calibration, a calibration gas of known composition is analysed. The gas chromatographs (GCs) analyse the sample and the components of the composition generate peaks in the output of the detectors. The area measured under the peak is divided by the known gas molar percentage of that component to derive a response factor for that component. That is, the response factor RF is calculated as follows:RF=Peak Area/Gas mole %  (Eq.1)
During normal analysis of an unknown sample, the response factor RF is used to calculate the unknown gas mole percentage of each component from the measured peak area and the response factor, according to:Gas mole %=Peak Area/RF  (Eq.2)
Gas chromatographs (GCs) may be delivered from a factory with a multilevel calibration already programmed. The multilevel calibration is performed on a number of separate gas samples corresponding to the compounds that the gas chromatograph is configured to detect. The multilevel calibration establishes the ability of the GC detector to measure a specific component and a response factor curve which is measured over a specified range of each component's concentration. The multilevel calibration also establishes the repeatability of measurements of each component over a specified concentration range. While this is an effective method to handle the linearity of the detector, many sets of gases at varying concentrations are required to obtain the multilevel calibration parameters. It is common for component parts of the GC, such as columns, diaphragms, detectors, etc. to be changed on site, after which the GC may require a new set of multilevel calibration parameters. For a number of reasons it is not always practical to perform multilevel calibration on site or in the field, not least because of the time consuming nature of a multilevel calibration process.
Other calibration techniques are used in the field. For example, a periodic auto-calibration may be performed using a certified gas sample mixture to ensure that the GC is functioning within a defined specification. The frequency at which the calibrations are performed is determined by the stability of the GC calibration and may for example be daily, weekly or monthly. However, if the GC does not provide a linear response or if it becomes necessary to use a certified gas with a different composition, errors may be introduced.
On-line (in the field or on-site) gas chromatography is now commonplace in the North Sea and is frequently used within Fiscal and Custody Transfer measurement systems. The uncertainty of the analysis from the gas chromatograph (GC) is of the utmost importance with the resultant analysis frequently at the core of economic transactions [1].
In recent years there has been increased interest in condition based monitoring (CBM) and in-situ verification of measurement devices. For example, publications can be found on ultrasonic meters [2], leading last year to the DECC policy statement outlining generic minimum requirements that would allow CBM to be considered. Similar assertions are being made regarding on-line verification of on Coriolis Meters [3]. Orifice plates have also recently seen significant effort put in to the development of diagnostic capabilities [4] Error! Reference source not found.
However, surprisingly, it would be reasonable to state that although the correct functioning of the GC is critical in today's measurement systems, comparatively little attention has been paid to verification and monitoring strategies [5, 6, 7, 8]. Although the modern gas chromatograph is an extremely repeatable device there remain several fundamental issues. For example, if the valve timing in the GC is wrong or drifts over time a systematic error can ensue. A recent study performed by the applicant identified a situation where such an error was present, and if it remained undetected, would have resulted in an on-going error in the calorific value of natural gas of up to 1.4%. For a typical production volume of gas of 3 to 4 million m3 per month and taking an example gas price of $90 per 1000 m3 this error would equate to a value of around £270,000 to £360,000 per month. Under the assumption of suitable sampling and conditioning the uncertainty of the GC measurement is generally driven by the linearity and the repeatability of the GC and the quality of the (certified) reference gas mixture. Various methods presently exist which may be used to obtain the GC repeatability. ISO 10723 [9] describes a method of performance evaluation using multiple calibration gas compositions to obtain the linearity of the GC as well as its repeatability. ASTM D1945 [10] provides a standard test method for the analysis of gas with a GC with stated levels for the expected repeatability and reproducibility.
However, these methods are only valid as long as the GC maintains the performance characteristics measured on the day of the test. ISO 6974 [11] describes data processing for the tailored analysis of natural gas with the aim of defining the uncertainty in the mole fractions of the component measured. However there remains little practical guidance on how to implement Condition Based Monitoring of on-site gas chromatographs.
A common method used in industry utilises measurements obtained every few minutes over a predefined period (for example, every four minutes during a 48 hours period), to obtain an uncertainty value for the GC chromatograph. The composition of a natural gas reference sample is measured every four minutes during the 48 subsequent hours after a calibration with a different reference sample has been made. The measurements taken during the 48 hours period are used to calculate a value representing repeatability relative uncertainty of the GC and its measurements.
This method may be carried out when the GC is in the laboratory or at the factory and although it is not usual, it can also be carried out when a GC is on-site. The uncertainty value obtained in the factory or in the laboratory is used in conjunction with composition measurements taken when the GC is on-site long after the repeatability relative uncertainty value has been obtained. This repeatability relative uncertainty value might not be representative of the actual GC uncertainty after several days or weeks after the GC is installed on-site and therefore the economic transactions based on the measured natural gas compositions and the repeatability relative uncertainty value obtained in the laboratory or factory might be overestimated or underestimated.
It is amongst the aims and objects of the present invention to provide improved methods of analysing gas chromatography data, and in particular, improved methods of uncertainty monitoring for gas chromatography apparatus. Additional aims include providing methods of calibrating, monitoring and/or maintaining gas chromatography equipment. The invention presents a novel method for monitoring a GC whilst also providing an on-line estimate of the overall uncertainty in the natural gas composition measurements which overcomes or at least mitigates one or more drawbacks of the previously proposed monitoring and maintenance scheduling methods for on-site gas chromatographs.
Further aims and objects of the invention will become apparatus from the following description.